This study presents a robust approach for enhancing the thermal insulation, mechanical durability, and wash stability of organic cotton fabrics (OCFs) through a polydopamine-assisted silver nanoparticle (AgNP) deposition and subsequent thermoplastic polyurethane/silica aerogel/dimethylformamide (TPU/SAP/DMF) composite coating. A systematic investigation of silver-plating reaction time revealed its critical influence on surface morphology, roughness, and coating adhesion. Optimal AgNP growth occurred at 20 min, yielding a 94.9% increase in peel strength and a 91.7% rise in surface roughness. Infrared reflectance was significantly enhanced in the 8–14 μm range, with a peak reflectance of 16.0%, demonstrating the silver layer’s ability to reflect radiative body heat. The coated Ag/OCF composite achieved a skin temperature increase of 2.78 °C and a clo value (a standard measure of clothing insulation, where higher values indicate greater resistance to heat loss) of 0.20, a 122% improvement over untreated OCFs. Additionally, mechanical performance and durability were preserved after multiple washes, highlighting the stability of AgNP anchoring. These findings establish an effective multifunctional textile platform for smart thermal regulation, suitable for advanced clothing systems and adaptive insulation materials.